It was reported earlier by the Principal Investigator (Biochem. Biophys. Res. 53: 894, 1973) that there exists in a fraction of human blood serum a group of inhibitors of the insulin protease of rat liver which were not described earlier and which prevent circulating insulin from being degraded. Because of low yields and variability in the properties of the fraction, purification was attempted from whole plasma. The inhibitors in whole plasma have been found to be bound strongly to other proteins present from which they can be dissociated by salt fractionation and urea treatment. The inhibitors have now been purified 1000-4000 fold. In the work proposed one of the inhibitors will be purified to homogeneity, its properties studied in detail and submitted to a collaborator for the determination of the amino acid sequence and the latter will be compared with that of insulin.